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REPORT ON THE OCEANOGRAPHIC INVESTIGATIONS CARRIED OUT AT
THE SOFALA BANK BY THE SOVIET TRAWLER "SEVASTOPOLSKY RYBAK"
IN SEPTEMBER~DECEMBER 1982
by
Antonio Jorge da Silva
INTRODUCTION
In September-December 1982 a fisheries research cruise was carried out at
the Sofala Bank (figure 1) by the SRTM "Sevastopolsky Rybak".
Oceanographic investigations have been conducted in association with the
fisheries research.
Three coverages were made of the Sofala Bank each lasting for slightly less
than three weeks. This will allow for three descriptions of the oceano
g·raphic conditions in the area, with some reference being made to variabi
lity. This paper is just a preliminary report of those investigations.
METHODS
The survey grids are shown in Fig·ures 2-4. No attempt was made to
indicate which observations were carried out at each station, as this would
make the figures unnecessarily complicated. Instead, the positions of the
stations used for the different horizontal distributions are indicated by
dots in Figures 5-12.
Nansen casts for temperature and salinity determinations were performed
at virtually all stations during the first and the third coverages. Water
bottles were placed at 0-2-3-5-10-20-30-50-75-100 m. Additionally,
bathythermograph soundings were carried out to the bottom at all stations
during the three coverages. Secchi depth and water colour (Forel Scale)
were determined at all stations performed during daytime in the first
coverage.
For sake of simplicity water colours were grouped in four categories:
1
2,3
4,5,6,7
8,9
deep blue
greenish blue
bluish green and green
yellowish green
Temperature and salinity profiles are shown in Figures 13-28. The
sections were numbered from south to north using the third coverage as a
basis. Each figure refers to one section and contains the three profiles
corresponding to the three coverages (e.g. Figure 16). In cases where
one or two profiles were missing the corresponding spaces were left blank
(e.g. Figures 13, 14 and 21). Surplus sections from the first coverage
were not considered.
RESULTS
During the approximately two months period separating the first from the
third coverage a temperature rise of about 2°C has occurred. In each
coverage the temperature tended to be some 2. 5°C higher at the north
than at the south (Figures 7, 9 and 11). No similar trend was found in
what concerns salinity, although important changes were observed from
one coverage to another (Figures 8, 10 and 12). It is quite remarkable
the correspondance between the salinity distribution of Figure 8 and the
distributions of optical properties of Figures 5 and 6. This suggests that
water of river origin can successfully be traced by means of colour sensors.
Important differences in the surface characteristics allow us to distinguish
three main areas in the Sofala Bank:
A - north of Pebane (latitude 17° 20'S)
B ·- between Pebane and roughly latitude 20°S
C - south of latitude 20°S
Area A was always characterised by the presence of salinities between
35, 1 and 35.3, values that are not typical of shelf water. During the first
coverage values above 35,2 were only observed north of latitude 17°S
(Figure 8) while, during the third coverage, they were found over the
whole shelf near Pebane (Figure 12). During the second coverag·e those
salinity values were instead found at the inner part of the shelf extending
to area B (Figure 10). Changes in the surface salinities from the first to
the third coverages were accompanied by the disappearence of an inshore
pocket of low temperature and its replacement by warmer water (Figures
7, 9 and 11).
Area B can be considered as characterised by a strong interaction of Low
Salinity Shelf Water (LSSW), mainly of Zambezi origin, with Oceanic Water
transported by the Mozambique Current 0 Strong horizontal salinity
gradients were therefore found in this area. Great variability was also one
of its characteristics 0
During the first coverage the lowest salinities were found to the north of
the Zambezi delta associated with a low temperature tongue extending
northwards (Figures 7 and 8). Salinities higher than 35 0 0 were only found
at the outer part of the shelf.
During the second coverage (Figures 9 and 10) only some remains of the
LSSW could be detected to the north of the Zambezi, near Quelimane.
Surface salinities higher than 35 0 0 were present over the whole shelf
north of the delta, while the lowest salinities were found to the south of
it 0 The salinity range and gradient were similar to those found during the
first coverage (Fig·ure 8). Similm· temperatures were detected at the inner
and outer parts of the shelf north of the delta, as well as over the whole
shelf due south of the Zambczi (Figure 9) 0 An area of lower temperature
was present at the mid part of the shelf north of the delta.
During the third coverage salinities greater thun 35. 1, characteristic of
Oceanic Water, were present over the whole shelf north of Quelimane
(Figure 12) o To the north of the Zambezi the temperature was fairly
constant around 28°C (Figure 11) 0 The lowest salinities were found near
the Zambezi mouth. Comparatively to the second coverage (Figure 10),
LSSW was apparently starting to propagate northwards (Figure 12). South
of the Zambezi the salinity distribution was similar to those of the two
previous coverages, except for an inshore pocket of higher salinity near
Beira. To the south of Quelimane the isohalines tended to be roughly
parallel to the isobaths at the outer part of the shelf. The temperature
distribution (Figure 11) shows a high temperature tongue extending
southwards at the inner and mid parts of the shelf and a low temperature
tongue extending northwards at the outer shelf.
Area C was characterised by the presence of high salinity water that is
known to be of shelf origin (BRINCA et al. 1981 a, 1981 b, 1983 a, 1983
b). This area is separated from Area B by a salinity front running more
or less zonally (Figures 8 and 12). A positive onshore salinity gradient
was observed during the first coverage (Figure 8), while Figure 12
reveals some kind of an eddy situation. A less intense meridional front
separated the High Salinity Shelf Water (HSSW) from the LSSW coming
from north.
Sections I~III (Figures 13-15) reveal that the HSSW was nearly
homogeneous from surface to bottom. Some unstability was, however,
detected at the surface during the third coverage (Figure 14-C). The
LSSW coming from north can also be traced in Figures 14 and 15.
Apparently, its influence in area D was greater during the first coverage
than during the third one.
During the third coverage the HSSW propagated northwards reaching the
area of Section V (Figures 16-C and 17-C). Traces of this water were in
fact found in stations 197, 201 and 204. At station 201 (Figure 17-C) it
was confined to the upper 3 m giving rise to an unstable situation. At
station 208 (Figure 18-C) it was still possible to trace the influence of
HSSW.
Contrasting with the nearly homogeneous vertical structure observed in
sections I-III, the other sections show vertical gradients over the shelf.
The temperature profiles reveal that water lifting occured at the outer
part of the shelf, which was probably a manifestation of a similar
movement over the non-investigated slope area. Water lifting was quite
clear in sections VIII and XII during the first coverage (Figures 20-A and
24·-A), sections VIII, XI, XII, XIII and XVI during- the second coverag·e
(Fig·ures 20-B, 23-B to 25~B and 28-B), and sections IV-IX, XI, XV and
XVI during· the third coverag·e (Figures 16-C to 21-C, 23-C, 2'1-C and
28-C). Particularly evident cases are those shown in Figures 19-C and
23-B to 25-B.
Salinity values greater than 35.3 can be seen in Fig-ure 22-C at dephts of
30-40 m. These values can only be due to lifting- of Subtropical Water from
about 150 m, where the core of the subsurface salinity maximum is usually
found offshore (SAETRE and JORGE DA SILVA, 1982). Figure 24-C shows
a similar situation where this water was lifted to the surface. In none of
these figures can one find any indications of water lifting in the
temperature profiles. This sugg-ests that this process occured before the
area was surveyed.
Salt wedges due to the influence of LSSW, particularly connected to the
Zambezi runoff, were observed mainly to the north of the delta. They
were quite clear during the first coverage in sections X and XII (Fig-ures
22-A and 24-A), changing into a distinct two-layer structure in section
XIII (Fig·ure 25-·A), and to an offshore lens of low salinity further north
in section XIV (Fig·ure 26-A). During the third coverage salt wedg·es were
found in sections VIII-X (Fig·ures 20-C to 22-C). Figure 19-A shows the
only case of a salt wedge found to the south of the Zambezi.
DISCUSSION
The results of the three coverages made during the present cruise may
reflect three different, successive stages of a variable dynamic process.
Each stage can be characterised by a certain degree of interaction of the
relatively warm and saline Oceanic Water, transported by the southg-oing
Mozambique Current, and the slightly cooler Low Salinity Shelf Water
(LSSW), mainly of Zambezi origin.
Lateral displacements of the core of the Mozambique Current may have
determined the routes of propag-ation of LSSW. During· the first coverage
only the shelf area north of Pebam} seemed to be directly influenced by
Oceanic Water, while the LSSW flew northwards and turned offshore
~ 10 -
between Quelimane and Pebane (Figures 7 and 8; see also Figures 24-A to
26-A).
During the second coverage the direct influence of Oceanic Water was
apparently exerted over the whole shelf area north of the Zambezi mouth,
and drastically reduced the northward propagation of LSSW (Figure 10).
This is seen to flow mainly southwards associated with temperatures
similar to those of the Oceanic Water (Figure 9). This association may be
regarded as an indication of the important role played by the Mozambique
Current in the transport of Zambezi water. Figures 23-B to 25-B provide
some evidence that the lower temperatures found over the mid part of the
shelf north of the Zambezi (Figure 9) were due to lifting of water at the
outer part of the shelf.
During the third coverage, while the whole shelf area to the north of
Quelimane showed surface values typical of Oceanic Water (Figures 11 and
12), low salinities were found to the north of the Zambezi apparently
associated with a tongue of low temperature that extended northwards,
The shape of the isotherms in Figure 11 suggests that such tongue was
probably the result of an intrusion of oceanic water onto the shelf area to
the south of the Zambezi. This might have forced part of the shelf water
northwards, while another part was forced southwards, as suggested by
the lower salinity tongue surrounding the area of HSSW, However, at the
inner part of the shelf south of the Zambezi the water was apparently
moving· southwards, as suggested by the shape of the isotherms and the
low surface salinity values.
The present results show that the southward transport of Zambezi water
is apparently a permanent feature. On the other hand, its northward
flow , although associated with lower salinities, seems to depend larg·ely on
the direct influence of Oceanic Water over the shelf area.
RECOMMENDATIONS FOR FUTURE RESEARCH
Having in mind all the results from the investigations carried out in the
Sofala Bank area (BRINCA et al., 1981 a, 1981 b, 1983 a, 1983 b),
attention should be payed to the following· points in future research:
- 11 -
i) Cruises covering· the whole area should always be
conducted in the same direction (preferably north- south),
particularly when surveys are made in sequence, as in the
present case;
ii) Water colour and Secchi depth should always be
determined aiming, inter allia, to obtain ground truth for
satellite data;
iii) Selected sections should be extended beyond the
slope and carried out in all research cruises (four sections are
probably enough, approximately along the directions of sections
II, VI, X and XVI);
iv) Short-term variability studies should be started, at
least along two of those selected sections;
v) Direct current measurements should be conducted over
the shelf in selected points along the sections where time
variability studies are carried out.
REFERENCES
BRINCA, L., SILVA, C. and SILVA, A. (1981 a) Relat6rio do cruzeiro
realizado no Banco de Sofala pelo arrastao "Muleve" em Julho/
Agosto 1979. Informacao No. 4, Instituto de Desenvolvimento
Pesqueiro, Maputo, 53 pp.
BRINCA, L., REY, F., SILVA, C. and SAETRE, R. (1981 b) A survey
on the marine fish resources of Mozambique, Oct. -Nov. 1980.
Reports on Surveys with the R/V Dr. Fridtjof Nansen, Insti
tuto de Desenvolvimento Pesqueiro, Maputo, Institute of
Marine Research, Bergen, 58 pp.
BRINCA, L., BUDNITCHENKO, V.A., JORGE DA SILVA, A. and SILVA,
C. (1983 a) A report on a survey with the R/V "Ernst
Haeckel" in July-August 1980 Shallow-water shrimp and by
- 12 -
catch; Oceanography. Revista de Investigac;;o Pesqueira, No. N I
6, lnstituto de Investigacao Pesqueira, Maputo, 105 pp. - I
BRINCA, L.,
SAETRE,
Bank
JORGE DA SILVA, A., SOUSA, L., SOUSA,
R. (1983 b) A survey of the fish resources
M. I. and
at Sofala
the R/V
Pesqueira,
15 pp.
Mozambique, September 1982. Reports on
Dr. Fridtjof Nansen, Instituto de
Maputo, Institute of Marine Research,
Surveys with N
Investigacao I
Bergen, 70 +
SAETRE, R. and JORGE DA SILVA, A. (1982) Water masses and
circulation of the Mozambique Channel. Revista de ' <N Investigacao
Pesqueira No. 3, lnstituto de Desenvolvimento Pesqueiro, Maputo, 83
pp.
- 13 -
Figure 1. Sofa1a Bank - bathymetry (m) and main rivers.
•) - Gauge station
~ 14 ~
35' 36' 37' 38' 39° 40°
Figure 2. Station grid,22 Sep-8 Oct 1982
21
17.
18.
2 o·
21'
QUELIMANE
;:126
130 ·1J1,' • '· .127 m(
• ~1~5 /
1 ~9 .. } ~0 ,136 -~"/ '141
,/ •142
\ •144
l
- 15 -
· ... ~-· (.· '111
~ ·.m
Figure 3. Station grid, 8-25 Oct 1982
QUELl MANE
35" 36" 37°
Figure 4. Stat ion grid, 18 Nov- 4 Dec 1982
•. ' /i~~ ;''i'os·
38"
38"
.-··
40"
39" 40"
16'
water colours
ld [21 2-3
11'
~ '~- -7
~ 8-9
QUELIMANE 18'
35. 36° 39°
Figurt> 5. Water colour (Fore I), 22 Sep- 8 Oct 1982
17° SO m
QUELIMANE •
19°
BEIRA
2 o·
21'
35' 36' 37' 38' 39' 40°
Figure 6. Secchi depth, 22 Sep-8 Oct 1982
- l7 -
17.
O.UELIMANE 18'
19"
zo·
21"
35" 36" J7• 39" 40"
Figure 7. Surface:' temperature, 22 Sep-8 Oct 1982
.! I
21° I ,i
35" 36' 37" 38" J9• 40°
Figure 8. Surface salinity, 2 2 Sep- 8 Oct 1982
- 18 -
35. 37° Js• J9•
Figure 9. Surface temperature, 8-25 Oct 1982
17. s PEBANE
QUELIMANE 18"
19°
2 o·
21°
,J (
JG• 37" Ja• 39" 40°
Figure 10. Surface salinity, 8-25 Oct 1982
~ 19 -
0
17 °
QUELIMANE
IS'
19'
21°
35• 37' 38• 39° 40°
Figure 11. Surface temperature, 18 Nov- 4 Dec 1982
17. 5
QUELl MANE 18.
19°
2 o·
21'
35' 36' 37• 38• 39°
Figure 12. Surface salinity, 18 Nov-4 Dec 1982
c 177 178 179 180
0 ,.. .... -- ... ,
,. I I I
10 , , I I \ I
\ 25jS \ \
20 I I I I I I I l I I I
Ill I I QJ 30 I ... I -QJ
E 40
t •c
50
60
20 -
0 20 40 -'----'--'---'---'---L
181 0
26
10
20
30 \
', ... 40
50
60
km
Figure 13
Temperature and salinity profiles
at Section I
C- 18 NOV 1982
177 178 179 180 181
~
A 0
24 24 !:::-:::-
211 210
>24 Vl 10 ilJ '-.... C!J
E 20
c 184 185 186 187 183 182
0 I I I I I
' ....... «:::!:26.0 --25.5
10 < 26
20
Vl ilJ 30 '--CIJ
E
40
50
60
21 -
j_ 10 1,0 ~·J..~_.J__ I
0
10
20
0
10
20
30
40
50
60
kIll
24 23 22 21 20
Figure 14
Temperature and salinity profiles
at Section II
A - 23 SEP 1982
C -18-19 NOV 1982
184 185 186 187 183 182
,( ,[ I
,J, I I
- 22
0
A 25 26 27 28 29
0 I I ;J' I
10 Vl
24 OJ < 24 ... o; 20 E
30 t ·c
c 188 189 190 191 192 193 194
0 I I I ' I ' I ' > 26 I ' 10 I ' I I I I
26 25.5 25~5 I I I I
20 I I I ' I ' I
Vl I OJ I ... 30 I .... I OJ I E
40 t ·c _ .... ~..-25
50
20 40
0
10
20
30
0
10
20
30
40
50
km
25 26 27 28 29
I I I
\ 34.8 35.1 34!9
I 35.0 I I I I I
s I I I
Figure 15
Temperature and salinity profiles
at Section Ill
A - 23 SEP 1982
C - 19-20 NOV 1982
190 191 192 193 I
\ I I
3)1
I
35.3 35.2
>35.3
194 I
35t
<35.1
5
c 195 196 197 198 199 200
0
10 2~ 20
Ill 30
" ... -" E 40 t ·c
50
60
- 2] -
0 20 40
0
10
20
30
40
50
60
km
Figure 16
Temperature and salinity profiles
at Section IV
C - 20 NOV 1982
200
J, \
s
~ 24 ~ 0 20 40 km
A 33 35 37 33 35 37
0 I
,( I
,1,
I 0
10 10
20 20 Vl llJ ... o:; 30 30 E
40 40
8 137 138 139 140
0 I I
) I I
>25 10
20 Figure 17 25 Vl llJ ...
Temperature and salinity profiles at -llJ
E 30 Section v
40 A - 24 SEP 1982
B - 24 ocr 1982
50 c- 21 NOV 1982
c 201 203 205 207 205 2Q7
0 I I I
\}:::27 I 0 I
10 10
20 6"'
20
30 30 Vl t ·c llJ ... o:; 40 40 E
50 50
60 60
70 70
- 25 -
0 20
c 208 209 210
0 211 212 213 214
10
20
Ill w 30 ... ..... w E
40
50
60
40
0
10
20
30
40
50
60
km
Figure 18
Temperature and salinity profiles
at Section VI
C - 2 2 - 23 NOV 198 2
210 211 212 213
\ I
s I I I I
214
3'L \
I
A
VI <II .... .... <II E
8
VI <II .... .... <II
E
c
VI <II '-.... <II
E
43
20
30
40
128
217
50
45
/.,.-- .... , ' I \
I I
\
' ' I
\
' 24·~
1 >24 ' ' \
·c
129 130
220
t ·c
- 26 -0
47 0
10
' \ '
131
23
216 215
26
70
km
Figure 19
Temperature and salinity
5
profiles at
ection VII
27 SEP 1982
B - 20 OCT 1982
A -
C- 23-24 NOV 1982
217
35:{
~
A 52 51 so
0
/-~~ I
10 I I I I
241.5
20 I I
' I I Ill
30 (lJ ... .... (lJ
E 40
so
60
B 124 125 126 a
10
Ill 20 (lJ ._ .... (lJ
E 30
40
c 221 222 2 23 0
28
10
27
20
Ill (lJ '- 30 .... (lJ
E
40
so
60
- 27 -
0 20 40
0
10
20
30
40
so
60
0
10
20
30
40
so
60
km
52 51 so I . ~ \ ) 'S 35.2 34;8 3~.0 3~.1
s
-,35·2
Figur<~' 20
Temp<~'rature and salinity profiles a
Section VIII
A. - 28 SEP 1982
B - 19 OCT 1982
C - 2S NOV 1982
221 222 2 23
B 123 12 2 121
0 I
25~5 I
10 I \ '- ... ___
20 Vl C!J ... .... C!J
30 E
40
c 2 24 225 226 227
0 I I 0 28 I
10
20
30 Vl C!J ... .... C!J 40 E
50
60
70
228 I
27
6
- 28 - 0 20 40 km __.___,__....___.___.__.._
0
10
20
30
40
50
60
70
Figure 21
Temperature and salinity profiles
at Section IX
B- 18-19 OCT 1982
C - 26 NOV 1982
224 225 226 227 228
n ' <35\ s
A 58 59 60 61
0 I I
~25~ 10
20 VI «< b.
q; E 30
24
40
23 50
c 229 230 231 232
0 I I
~28 10
VI 20 «< ... - 26 «< E 30
40
- 29 -
0 20 40
0
10
20
30
40
50
0
10
20
30
40
km
58 59 60 61
35·2
Figure 22
Temperature and salinity profiles
at Section X
A - 29-30 SEP 1982
C - 27 NOV 1982
2 29 230
-\)4.~ ~8 5.0 '> <::::::::: _ _; 5 .,
-:~ 35.2
20 40 km
8 116 117 118
0 26
10 ~' 24 Figure 23
20 Vl Q! ,_
Temperature and salinity profiles .... ~ 30 E at Section XI
itO
B- 16-17 OCT 1982
50 c- 28 NOV 1982
c 233 234 235 236 233 234 235 236
0 0
~ 10 10
20 27
20 > 35.1
30 30 Vl Q! .... .... Q! 40 40 E s
50 50 25
60 24 60
23
70 70
A
Vl <lJ '-q; E
B
Vl <lJ '-.... <lJ
E
c
til <lJ '....
0
10
20
30
40
0
10
20
30
40
50
60
70
0
10
E 20
30
- 31-0 20
74 75 76 77
t ·c
113 112
2
237 238 239 24 0
40 km
74 75 76 77 0
10
20
30
40
Figure 24
Temperature and sa 1 i nit y profiles
at Section XII
A - 3 OCT 1982
B - 15- 16 OCT 1982
c - 29 NOV 19 82
237 23 8 239 240 0
10
20
30
- 32 -
0 20
A 82 83
0
10 Ul <IJ ... "" E 20
30
B 108 109 110 111
0
10
20
Ul <IJ 30 .... ..,. <IJ E
40
50
60
70
c
Ul ,:l
<IJ .... +' <IJ
E 20
30
40
25
40
0
10
20
3
0
10
20
30
40
km
82 83 84
,.~34.0 ;::i?S2S
34.S
/ 3~.o ,
I
,' 35~1 I I s
Figure 25
Temperature and salinity profiles
at Section XIII
A - 4- 5 OCT 1982
B-14-15 OCT1982
C - 30 NOV 1982
241 242 243 244
B 107 106 105
0
10
Vl 20 Figure 26 OJ '-....
profiles OJ Temperature and salinity E 30
23 at Section XIV
40 A - 5-6 OCT 1982
B - 14 OCT 1982
c- DEC 1982
c 245 246 247 245 246 247
0 0
10 10
20 20
Vl OJ ... .... 30 30 OJ
E
40 40
- 34
0 20 40 km
B 102 103 104
0
10
Vl 20 Figure 27
"'' ... .... Temperature and salln1ty profiles <IJ
E 30 at Section XV
40
B - 13- 14 OCT 1982
50 20 c - 1- 2 DEC 1982
c 2 50 2 49 248 250 249 248
0 0 27~;
10 10
Vl 20 20
<IJ .... -<IJ
E 30 30
40 40
50 50
- 35 0 20 40 km
c 251 252 253 251 252 253
0 0
10 10
20 20
30 30 1/l <1J
.::: <1J 40 40 E s
50 50
60 60
70 70